This kind of holographic recording method and apparatus includes ones in which digital information to be recorded is converted into a two-dimensional bitmap pattern and this pattern is given to an object beam through light intensity modulation so that it is recorded as a data page.
Here, the bitmap pattern is typically rendered in light intensity modulation by using a spatial light modulator. Since this spatial light modulator is low in recording density as long as it intensity-modulates the object beam in two levels of ON and OFF pixel by pixel, attempts have conventionally been made to improve the recording density and data rate by means of a gray scale (so-called multilevel recording).
Among the methods of creating a gray scale as mentioned above are one for dividing the contrast of the intensity modulation, one for dividing exposure time, and one combining these.
Moreover, as described in G. W. Burr et al., Opt. Lett. 23 (15) 1218 to 1220 (1998), a gray scale such as mentioned above can be applied not only to bit-by-bit digital data recording such as in DVD (Digital Versatile Disc), but also to page type data such as that of a holographic memory.
Furthermore, since a laser beam emitted from a laser light source typically has a near Gaussian intensity distribution within its beam diameter, an object beam to be propagated through an object optical system also has a Gaussian distribution.
Consequently, the object beam immediately after intensity-modulated by the spatial light modulator decreases in intensity with increasing distance from the beam center as shown in FIG. 9.
When such an object beam is used to record a data page on a holographic recording medium, a similar image is reproduced by an image pickup device at reproduction time. This requires that the superimposed Gaussian distribution be electrically corrected after the detection of the image.
Nevertheless, since image pickup devices have fixed noise independent of detection intensity, there is the problem that emphasizing darkened pixels can also emphasize the fixed noise with a drop in the SNR of the image.
In this respect, there has heretofore been proposed a technique called apodization in which the object beam is shaped before irradiation from the spatial light modulator, as described in H. J. Coufal et al., “Holographic Data Storage,” Springer-Verlag (2000), pp. 369-381.
With the method of dividing the contrast of intensity modulation, which is one of the methods of creating a gray-scale mentioned above, the spatial light modulator, or the means for intensity modulation, is limited to devices of polarization control type (such as a liquid crystal display). Spatial light modulators of direct reflection type like a DMD (Digital Micromirror Device: Tread Mark) cannot be used.
A second problem of this method consists in that the gradation display requires fidelity of high precision. The image transfer from the spatial light modulator to the image pickup device also requires a pixel-by-pixel resolution. Afterimages and blurs of liquid-crystal spatial light modulators thus have a significant impact on crosstalk between the gray-scale pixels.
In reality, this method requires that the amounts of light for the pixels to propagate be modulated at high speed. This means the problem that devices containing analog elements such as liquid crystals cannot follow the high speed operation but be affected by afterimages and blurs.
Moreover, the apodization technique described in H. J. Coufal et al. above is one for redistributing beam intensities of Gaussian distribution into an intensity distribution that is step-functionlike in the radial direction of the beam by using an optical part such as a lens. There is a problem, however, since optical parts are expensive and can limit the degree of freedom of the optical system.